1. Field of the Invention
The present invention relates to the electrical supply for an electrochromic cell, notably a pane with electrically controlled light transmission. The invention is applicable, notably, to all electrochromic cells of large dimensions intended, for example, for controlling the solar input in a building or in a passenger space of a vehicle.
2. Discussion of the Background
An electrochromic cell is constituted of a stack of layers comprising an electrochromic material, that is to say a material capable of inserting, in reversible manner, cations, notably protons or cations of alkali metals and having coloration states that vary between a colored state and a decolored state, an ion conducting electrolyte and a counter-electrode which serves as a reservoir for the cations and which, like the electrochromic material, must be capable of inserting and de-inserting the cations, symmetrically with respect to the film of electrochromic material. In the case of a system functioning in transmission, such as for example a pane, the counter-electrode must also be suitable for having a decolored state when the film of electrochromic material is itself in the decolored state. These conditions mean that in general pairs of cathodic and anodic electrochromic materials are chosen, with for example a cathodic material such as tungsten oxide WO.sub.3 which becomes blue in color in the inserted state in association with an anodic material, such as iridium oxide IrO.sub.2 or nickel oxide NiO, which is decolored in the inserted state.
The film of electrochromic material must, furthermore, be in contact with a transparent electrically conducting film. The same is true for the counter-electrode, although in this case the transparent character is necessary only for systems operating in transmission. The stack is sandwiched between two substrates, that on the electrochromic material side being of necessity transparent, such as for example glass plates.
In order to make the system operate it is necessary to apply, between any pair of facing points on opposite sides of the electrolyte, a potential difference at least equal to the difference in the thermodynamic potentials of the desired insertion/de-insertion reaction. In practice, taking account, notably, of the interface problems and of the resistance of the electrolyte, the minimum value to be applied is always slightly higher than the difference of the thermodynamic potentials. The greater the potential difference applied, the more rapid the coloration or decoloration. Nevertheless, the operating voltage should not be too high, because it is important not to exceed the voltages that permit parasitic reactions such as, for example, the release of hydrogen in the case of a proton system. For each change of coloration state of the electrochromic system, it is thus necessary not to exceed a certain potential difference, hereinafter termed the limiting potential of the system. To give some idea, in the case of a cell of the type WO.sub.3 /proton electrolyte/IrO.sub.2, the limiting potential is +1.6 volts in coloration and -0.6 volts in decoloration (the signs conventionally adopted being (+) for the coloration voltage, (-) for the decoloration voltage; the limiting voltage should therefore be considered as an absolute value).
The electrically conducting films of the system are intended for the transfer of the charges, and the voltage only has to be applied between two diametrally opposite terminals of the electrochromic system. But it will be self-evident that the electrically conducting films of necessity have a certain resistance. Thus, in the case of a transparent film based upon indium oxide doped with tin (ITO), a square resistance of the order of 5 Ohms corresponds today to a conductivity optimum in industrial production conditions, and thus the ohmic drops of the system become greater as its dimensions increase. For this reason, the potential difference effectively applied between two facing points becomes smaller the further these points are from the terminals, and therefore for these points there is a very great delay in the coloration, the maximum coloration being obtained only after a period of several minutes. When the physical size of the system becomes large (for example of the order of a square meter), complete switching of the system may even prove impossible to achieve.
To overcome the problem of ohmic drops, the current leads are systematically made, not of point terminals, but of highly conducting strips or wires, for instance of copper, which extend along two opposite sides of the cell, so that all the points of a film equidistant from one of these sides are equipotential. Nevertheless, this is far from satisfactory once the width of the electrochromic system increases and exceeds, for example, 10 cm, a limit which is obviously incompatible with the production of a building pane or, for example, an opening roof for an automobile.
In European Patent Application EP-A-408 427, it has been shown that the switching speed of an electrochromic system is substantially improved if the potential difference applied is modulated during the switching period in such a way that the potential difference between a given point of the film of electrochromic material, chosen in immediate proximity to the electrical supply strip, and its facing point of the counter-electrode remains throughout this switching period lower than the limiting voltage at which parasitic reactions take place. This known method of supply leads to the application, at the start of a switching cycle, of a voltage which is much higher and it results in a speed of coloration or decoloration which is very substantially improved, for example six times higher for a pane width of 30 cm.
Nevertheless, for a width, for example, of one meter the switching times are still several minutes, at any rate if a satisfactory contrast is desired, for example of the order of 4, contrast being defined as the ratio of the light transmission in the decolored state to the light transmission in the colored state. Moreover, the supply process known from EP-A-408 427 has a tendency to accentuate greatly the contrast established at the start of switching between the edges of the pane and its central region. Ratios of more than 1 to 2 are commonly obtained, which corresponds to differences that are very easily perceived by the eye.